An image sensor is a semiconductor device that converts an optical image into an electrical signal. The fabrication process is very important in determining the performance of an image sensor. Two important processes are the color filter array (CFA) and microlens (ML) formation processes.
In a microlens formation process, there are typically gaps formed between neighboring microlenses. These gaps cause the image sensor's performance to suffer. Several studies and simulations have demonstrated that when the gaps are small, the photo sensitivity of the image sensor can be improved by more than 15%.
A typical microlens formation process in the related art uses an organic material in the form of a photoresist (PR) in which a thermal reflow is possible using thermal energy. In particular, a pattern is first formed at a position where a lens will be disposed by performing a lithography method using photoresist material. Next, heat is applied to the material to reflow it so that a spherical curve is formed. Then, it is cooled, thereby completing the fabrication of a lens.
In a typical microlens formation process, the width of the gaps between the microlenses is determined by a pattern formed through photolithography before reflowing the material. Accordingly, the minimum gap width is often limited to about 50 nm due to the current limitations of lithography resolution. Furthermore, when the material is reflowed so that the gaps between the microlenses are narrowed to less than 50 nm, neighboring microlenses may merge together during the reflow process, raising the probability that lens bridges will form. Thus, current microlens formation processes cannot form a microlens having a complete zero gap.
Lens bridges occur because of the mixing phenomenon present when hydrophobic materials come in contact with each other, such as when the hydrophobic photoresist comes in contact with a neighboring hydrophobic lens. This phenomenon is similar to when two water drops come in contact with each other on a glass window and are drawn together. As shown in FIG. 1, since the photoresist for forming the lens has a predetermined degree of viscosity in the fluid state, a gentle bend is generally formed between the photoresists 1 for forming the lens, thus generating a lens bridge.